Vaccines or Titers?
Susan Yohn, DVM, MS, DABVP Canine/Feline, DACVIM
Vaccine products have been used in companion animal medicine for more than 40 years. They could arguably be considered one of the most important scientific advancements that have helped us improve quality and longevity of the life of our cat and dog patients. In the last decade, a significant change in our thinking about vaccination protocols has been seen due to several different factors. Our understanding of the science of vaccination-induced immunity has evolved suggesting more prolonged infectious disease protection with specific core vaccines (canine distemper, parvovirus, adenovirus; feline panleukopenia virus). Major veterinary associations (AAHA and AVMA) with the support of vaccine manufacturers now recommend reassessment of annual vaccination protocols, especially for the core vaccines. Vaccination every three years has become accepted practice by many veterinary practices. Vaccine manufacturers have introduced new products with more extended duration of immunity and fewer antigenic components.
Our society has become more concerned about possible vaccine reactions in children, and this has also created concern over side effects in pets. Side effects are fortunately infrequent in dogs and cats, but there has been association between vaccination and the development of vaccine sarcomas in cats and immune-mediated disease in dogs. With the concern from pet owners about “over vaccination” and the possibility of serious vaccine side effects, clients may now be requesting vaccine titers instead of vaccines.
How reliable are these vaccine titers?
Vaccine titers have been the cornerstone of the change in our knowledge about vaccine protection. They measure the presence of serum antibody able to neutralize the virus and prevent infection. IgG is the antibody class measured by vaccine titer assays. Studies have shown that for specific viral diseases in dogs (parvovirus, distemper, adenovirus) and cats (panleukopenia virus) protection after initial appropriate vaccination can last for 4 to 10 years based on serologic titer results and challenge studies.
The “gold standard” assays for these virus titers are the viral neutralizing titer (VIN) and hemagglutination inhibition (HI). Most state diagnostic labs utilize the gold standard methodologies. Most, if not all, commercial veterinary laboratories have qualified and standardized alternative methodologies including enzyme immunoassays (ELISA) and immunofluorescence assays (IFA) to provide similar titer information compared to the gold standard VIN and HI assays. Due to variations in lab assay methodologies, different reference ranges, and potential change in titers from the pet over time, titers should not be compared between different laboratories. In-clinic titer test kits for canine distemper, parvovirus, and adenovirus and feline panleukopenia virus are now available. TiterCHEK® (Zoetis) and VacciCheck® are the names of two commercially available in-clinic titer kits. Both test kit systems have been validated independently and correlated with the gold standard titer assays.
Rabies virus titers can also be assayed by the fluorescent antibody virus neutralizing titer, but the assay is available only through a limited number of certified laboratories.
How are these titers interpreted?
Depending on the individual lab and test methodology used, vaccine titer results may be reported as “protective” or “positive” (high antibody titer), “not protective” or “negative” (low antibody titer), or “borderline.” Some assays will provide an actual titer level from the laboratory to use in interpretation. The in-clinic titer test kits provide either a protected or not-protected result (TiterCHEK®) or a semi-quantitative score for serum antibodies (VacciCheck®). Protective, positive or high titers indicate that there is a high antibody titer to that virus either from vaccination or previous natural exposure or disease. For pets with a not protective, negative or low titer, lack of virus protection is of concern and booster vaccinations are recommended.
The best correlation between antibody and protective immunity are for the viruses previously discussed (canine parvovirus, distemper, adenovirus, feline panleukopenia, rabies).
It is important to remember that other factors may affect titer interpretation and risk for the pet. The immune system is complex. Antibodies target the foreign invaders for destruction, but other immune system cells actually destroy the invader. An insufficiency in the cell-mediated immune system (immunosuppression) may leave a pet at risk for disease even if protective titers are present. Even if a titer is considered low, exposure to the same antigen may result in a rapid immune response with new antibodies produced within hours to days. These animals may be susceptible to infection but may be fully protected due to immune-cell memory and cell-mediated immunity. By law, rabies vaccine titers cannot be used in place of re-vaccination. When a rabies vaccination is not current, a pet that bites someone must be considered unvaccinated and quarantined.
Antibody titers to vaccines other than the core viruses have limited or no value as “protective titers.” Antibodies may persist for a short time, and there is no correlation between serum antibody testing and protection. This includes antibody testing for leptospirosis, feline herpes virus, feline calici virus, Lyme disease, and Bordetella. With other diseases, feline immunodeficiency virus for example, titer levels equate only to exposure to the pathogen or active infection.
What is the appropriate clinical application of these titers for the veterinary patient?
Protective titer levels may be appropriate clinically for specific patients. The most common application of vaccine titers is to help determine the need for vaccination in an adult pet. The reason for the request for titer assessment is often because of previous history of suspected or severe vaccine reactions in the pet, concurrent medical illness or just the owner’s concern about the need for vaccination. Evaluation of vaccine titers may be helpful to determine the need for core vaccines in other clinical scenarios:
- Evaluation of vaccine response in a young pet to help identify non-responders that may not be able to produce a protective antibody response
- Identification of possibly unprotected pets with recent exposure to a contagious viral disease
- Determination of antibody level in pet with unknown vaccination status
- Management of infection risks and infectious disease outbreaks in shelters
Vaccine titers may be helpful in determining the need for specific vaccines in the individual pet, but it is important that the owner understand the factors other than titer results that may affect the decision to vaccinate. Each veterinarian must determine the appropriate vaccination schedule for a patient based on risk of exposure, age of the pet and current medical status of the pet.
Vaccine titers should not be considered a substitute for an appropriate core vaccine protocol for most clinically healthy pets.
The recent outbreak of measles in children exposed to the virus at a national amusement park was suspected to be correlated with a decline in vaccination for this virus. This is a warning to us all and a reminder of the importance of consistent vaccination for “herd health.” All puppies and kittens should complete the recommended core vaccination protocols recommended with the last vaccine being at or beyond 16 weeks of age. An evaluation of pathogen exposure risk, lifestyle, age of the pet and concurrent medical problems along with vaccine titer status are all factors that should affect the decision to vaccinate the individual adult pet.